1. Field of the Invention
The present invention relates to an image forming device which is structured in such a manner that a main body control part for controlling an image forming part is connected, via a serial communication line and a plurality of select signal lines for designating a transmission target, to a plurality of paper feeding control parts which individually control a plurality of paper feeding cassettes that supply recording papers to the image forming part, thereby allowing the main body control part to control communications with each of the paper feeding control parts with a polling system.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 2000-351252 proposes communication means which can secure communications between a device main body and each additional device regardless of combinations of the additional devices that are loaded to the device main body, when option units as a plurality of additional devices are loaded to an image forming device such as a digital copying machine.
With this, combinations and orders of paper feeding cassettes can be set freely, when loading the paper feeding cassettes to the device main body as the additional devices, for example.
Hereinafter, the communication means depicted in Japanese Unexamined Patent Publication No. 2000-351252 will be described in details. Each of the paper feeding cassettes loaded to the image forming device includes communication means for transmitting and receiving signals to/from an image forming device main body or to/from the other paper feeding cassettes, a first connector, and a second connector.
The communication means transmits a signal that is received from the image forming device main body via the first connector to another paper feeding cassette via the second connector, and transmits a signal that is received from another paper feeding cassette via the second connector to the image forming device main body via the first connector.
Further, the communication means includes a return path for transmitting the signal, which is received from the image forming device main body via the first connector, to the image forming device main body via the first connector without transmitting it to the other paper feeding cassettes via the second connector.
Normally, the polling system is employed when the image forming device main body performs communications with control parts of each of the paper feeding cassettes via such communication means. That is, the image forming device main body transmits a control instruction individually to the control parts by performing polling in order from the control part of the paper feeding cassette loaded on a highest drawer to the control part of the paper feeding cassette loaded on a lowest drawer.
As shown in FIG. 1A, a control part of each paper feeding cassette 7 as the option unit executes drive or stop control of own paper feeding roller 71 and its transporting roller 72 disposed nearby for transporting recording papers. For example, a control part of a paper feeding cassette 7a controls a paper feeding roller 71a and a transporting roller 72a, and a control part of a paper feeding cassette 7b controls a paper feeding roller 71b and a transporting roller 72b. 
Each transporting roller 72 is disposed on a common transporting path, so that the control parts of the paper feeding cassettes 7 on an upper-drawer side, i.e., on a downstream side, need to execute drive or stop control of the corresponding transporting rollers 72 in order to transport the recording paper fed from the paper feeding cassette 7 on a lower-drawer side, i.e., from an upstream side.
However, when the communications between the image forming device main body and each paper feeding cassette are performed with the polling system described above, a necessary control signal is transmitted from the image forming device main body to the control parts of each paper feeding cassette in order according to the polling. Thus, the control signal required to be controlled simultaneously by each control part is received with a delay by a length of time that is required for the polling.
For example, in a case where the image forming device main body performs polling from the control part of the paper feeding cassette loaded on the highest drawer towards the control parts of the paper feeding cassettes on the lower drawers in order, there is a delay time of several msec. to several tens of msec. generated until the control signal is transmitted to the paper feeding cassette of the lowest drawer.
Therefore, following issues may be generated because of shift generated in driving timings and stopping timings of each of the transporting rollers 72.
For example, when a stopping control of the transporting roller 72 disposed on the upstream side of the transporting path becomes delayed from a stopping control of the transporting roller 72 that is disposed on the downstream side, a paper may be sagged and bent between the transporting rollers.
Further, when a driving control of the transporting roller 72 disposed on the upstream side of the transporting path becomes delayed from a driving control of the transporting roller 72 that is disposed on the downstream side, a tip side of a recording paper held at the transporting roller 72 on the downstream side is pulled by the transporting roller on the upstream side. This may generate a noise or skew (oblique movement) of the recording paper, which may result in a paper jam or damage to the recording paper.